Fluidised Bed Dryer (FBD) works on the principle of fluidisation, a process in which hot gas or air is introduced into the spaces between solid particles. Upward forces on the particles increase as the air velocity increases causing them to equal the gravitational forces below. A state of fluidisation now exists as the particles are suspended in what appears to be a ‘boiling bed’ of fluid. Each particle is in direct contact with, and surrounded by the hot gas or air – creating an efficient and uniform drying process. Some fluid bed has vibrating motors, to reduce the risk of ‘dead zone’ formation. ‘Dead zone’ is an area that does not experience fluidisation. FBD particles have high surface area per unit volume to enable fast drying, resulting in high mass and heat transfer. Final moisture content can be lower than 7% for good shelf life of dried products.
In the food industry (relative to non-convective solid-based drying):
Advantages of fluid bed
- < 3 MJ of energy used per kg of water removal, or <0.84 kWh of energy used per kg of water removal;
- Adaptable to work flow: Can run as batch or continuous processing;
- Lower operating cost per unit production (high rating in energy efficiency): This convective drying process is better than non-convective type drying, e.g. drum dryer, vacuum dryer. In other words, the operating cost is lower due to less energy utilisation per unit production.
- High productivity;
- FBD: 1 ~hr drying time
- Hot non-convective drying: around 8 hours drying time
- Freeze drying: >24 hours drying time
- Shorter drying time leads to less oxidation of bio-active compounds with better colour preservation.
Disadvantages of fluid bed
- Does not work on highly moist and aggregated solid particle
- Only works on non-adhesive and non-sticky solid particles